Sorbitan monostearate may sometimes be used in a carbon black reinforced tire sidewall rubber composition to enhance its surface appearance by migration of the sorbitan monostearate from within the rubber sidewall rubber composition to create a relatively glossy protective film on the visual surface of the tire cured rubber sidewall.
However, sorbitan monostearate is hydrophilic in nature and attracts water to thereby typically significantly reduce surface building tack of the uncured sidewall rubber component, particularly in a humid atmosphere or as a direct application of cooling water onto an uncured sidewall rubber component in a tire manufacturing procedure. Such humid atmosphere, or application of cooling water, is usually applied to the uncured rubber component immediately after it is formed by, for example, a rubber extrusion process.
It is an aspect of this invention to use sorbitan monostearate in a tire sidewall component in combination with a specialized tackifying agent to substantially maintain surface building tack of the tire sidewall rubber component in the presence of moisture-containing hydrophilic sorbitan monostearate film on the surface of the uncured rubber component.
For the cured rubber sidewall component, the rubber sidewall may tend to become brown in color as a result of migration of antidegradants to the surface of the rubber composition, together with exposure to atmospheric ozone, ultraviolet light and weathering in general and the presence of the sorbitan monostearate film on the cured sidewall rubber surface tends to mask the presence of various antidegradants and tend to mitigate the effects of exposure to such atmospheric conditions to thereby enhance the appearance of the tire sidewall surface. Such phenomena are well known to those having skill in such art. For example, see U.S. Application Publication No. 2003/0205309.
It is to be appreciated that this invention is intended to go beyond a simple reduction or substantial elimination of a surface effect of one or more offending ingredients and exposure of the sidewall to atmospheric conditions.
Indeed, an important aspect of this invention is an inclusion of the sorbitan monostearate and its accompanying slow migration to the cured sidewall rubber surface in combination with a specialized tackifying resin to substantially retain building tack for the uncured rubber sidewall surface in the presence of moisture.
In the description of this invention, the term “phr” where used relates to parts by weight of an ingredient per 100 parts by weight of rubber, unless otherwise indicated.
The terms “rubber” and “elastomer” are used interchangeably unless otherwise indicated. The terms “vulcanized” and “cured” are used interchangeably unless otherwise indicated. The terms “compound” and “rubber composition” may be used interchangeably unless indicated.
The term “building tack”, or “surface building tack” of the surface of an uncured rubber composition relates to surface tack present on the surface of the uncured rubber tire component which promotes adhesion between uncured rubber components when they are combined in the building of an uncured rubber tire. In this manner, the assembled uncured tire rubber components remain together prior to the curing of the tire assembly of rubber components. Such building tack and its use is well known to those having skill in such art.